Several characteristics typical of horse’s reproductive philosophy together with some technical limits have slown down the spread of the use of this reproductive technique in horses compared to what happened in cattle industry. Because of its high cost, embryo transfer is a reproductive technique reserved to animals with a high genetic value. The success of this technique is determined by the careful selection of the donor and of the recipient.
The age, the choice of the stallion, the guidelines of breed books, the number of estimated pregnancies and the potential value of the foal are the fundamental aspects to consider and the most important is the careful selection of the recipient.
In embryo transfer programs, most of the donors is formed by old subjects hypofertile since broodmares reach the greatest potential value as subjects of reproduction in a rather advanced stage of their life, with the exception of young subjects used in sports competitions who otherwise wouldn’t have the faculty to reproduce and breed a foal.
The recipients should be of a suitable height and weight, have normal
estrous cycles(1) and should not have ovarian and uterine pathologies. The success of embryos collection is influenced by the number of ovulations, the age and fertility of the broodmare, the day of collection and the quality of the stallion’s semen.
Embryos are generally flushed from the uterus of a donor mare on the 7th or 8th day after the ovulation. If the same operation is carried out on the 6th day is associated to a lower percentage of success and for this reason it’s mainly reserved to those embryos which are destined to be frozen because it has been proved that the
cryopreservation(2) has a positive result when it’s carried out on small embryos 6 days old.
The decisive factor for the recovery of embryos is the fertility of the donor mare, old mares with
anamnesis (3) of hypofertility produce less embryos while the absence of an embryo can be the consequence of a series of pathologies.
Most donor eggs are inserted into a recipient mare's uterus trans-cervically, through an artificial insemination straw inserted through the cervix. Eggs can also be implanted surgically.
The introduction of equine frozen embryos transfer represents one of the main progresses in “commercial” embryo transfer. The freezing of embryos at 5°C allows to send them to an embryo transfer centre where they are implanted in the recipient broodmare.
Another cost of producing pregnancies using embryo transfer comes from the maintenance of suitable recipient mares before transfer. Many veterinarians and horse’s owners are not willing to invest capitals to acquire and take in the required number of recipient broodmares. There are several embryo transfer
centers that accept embryos which are sent to be implanted in recipient broodmares. It doesn’t seem to be any difference in the percentage of pregnancy between embryos that have been frozen (for later use) and transferred (up to 24 hours) and fresh embryos that are immediately transferred.
Article written by Peter F. Daels
"Embryo transfer: transported embryos in Europe?"
(1)The mare presents seasonal estrus cycles with more frequency and regularity from the end of February until June. The length of the cycle is 21 days (20-24). Estrus can last from 1 to 10 days, ovulation appears towards the end of heat. During her cycle, the mare is nervous, she urinates frequently and when she lifts her tail she presents a typical and rhythmic movement of vulvar labia.
(2) Maintenance of embryos vitality thanks to a considerable and controlled fall in temperature.
(3) A detailed collection of all the news concerning the mare, her life, her past diseases, the beginning and the course of the illness in progress. It’s of great importance to orientate the veterinarian’s diagnosis.